The invention is directed to low gold dental alloys, especially for firing on dental porcelains, the alloys containing 20 to 35 wt.% gold and 45 to 65 wt.% palladium.
For a long time there have been used in denistry crowns and bridges which have a metallic core and which are coated with a tooth colored ceramic. Such a combination compared to pure metallic crowns and bridges has advantages, both esthetically and physically, which result from the low heat conductivity of the ceramic. There have proven good for this purpose alloys having a high gold content which contain about 60 to 90 wt.% gold, about 1 to 15 wt% platinum, about 1 to 15 wt.% palladium, as well as silver, tin, indium, gallium, iron, and fine grain forming elements such as ruthenium, iridium, or rhenium. Because of the drastically increasing noble metal prices in the past years, these high gold content alloys have become extraordinarily expensive. For this reason and because of a general necessary reduction in cost in methods of health, therefore, there have been developed in recent times an entire series of low cost noble metal alloys which are suited for facing a dental ceramic. These include both alloys of reduced gold content based on gold-palladium or gold-palladium-silver with about 45 to 55 wt.% gold and also alloys based on palladium and palladium-silver which contain either no gold or only a few wt.% of gold.
Alloys for firing on dental porcelains must satisfy a number of requirements, such as, e.g., good castability, sufficient strength and ductibility, compatibility of the elasticity and thermal coefficient of expansion of the alloys with the commercial dental ceramic compositions, as well as a sufficient, but not too high, hardness. Furthermore, these types of alloys must possess a sufficient strength at the firing temperatures of the ceramic. Besides, there must not occur undesired reactions between ceramic and the oxide layer of the alloy, especially discolorations and the formation of bubbles.
While high gold content alloys for the most part satisfy these requirements, practically all known reduced gold and gold free noble metal firing alloys have a more or less severe defect. Thus, the commercial alloys on the basis gold-palladium with a gold content between 45 and 55 wt.% almost without exception have a very high melting interval which in part exceeds the capacity of commercial casting apparatuses. A further weaknes endangering the success of a dental ceramic operation with this type of alloy is its very low thermal coefficient of expansion of, in part, less than 14.0.times.10.sup.-6 K.sup.-1 which in unfavorable molding of the restoration or in subequent solder operations in faced metal ceramic parts can lead to cracks in the ceramic and also to spallings. The most severe disadvantage of these alloys, however, is in the inclination to form bubbles in the boundary surface between metal and ceramic after melting in the graphite crucible. The chief cause for this bubble formation is that the carbon taken up by the melt from the graphite crucible in the firing of the ceramic with oxides of the alloy or the ceramic burns to form carbon monoxide which remains behind as gas bubbles in the boundary surface (e.g., F. Sperner, dental-labor, Vol. XXX issue 12/1982, page 1733).
Alloys based on palladium are likewise associated with a large number of defects. For example, with these alloys the addition of alloying elements, which reduce the melting interval to the needed values lead to an unacceptable increase in hardness. With alloys based on palladium additionally because of their high palladium content and the susceptibility to carbon absorption associated therewith in no case can they be melted in a graphite crucible. The absorption of carbon would drastically increase the brittleness otherwise occurring with these alloys and besides make impossible a bubble free facing with dental ceramics.
In German OS No. 2944755 there are described numerous fired on alloys having about 32 to 63% gold and 29 to 58% palladium, which also contain indium and to which there can be added up to several percent of gallium, tin, copper, aluminum, titanium, and silver. The copper content in these alloys must be over 5%. However, it has been shown that even those alloys if they contain less than about 40% gold display very high liquidus temperatures (in part over 1400.degree. C.). A reduction of the liquidus temperature through variation of the remaining additives leads to impermissible hardness valves.
Therefore, it was the problem of the present invention to provide low gold dental alloys, especially for firing on dental porcelains from 20 to 35 wt.% gold and 45 to 65 wt.% palladium, whose gold content should be clearly below the customary gold-palladium economical fired alloys which can be easily worked, correspond to the known dental porcelains in their thermal coefficient of expansion, in which there do not occur discoloration of the porcelain or bubble formation after melting in graphite crucibles and which compared to the known economical gold alloys above all should display a reduced hardness and a decreased melting interval.